Aberrant excitatory rewiring of layer V pyramidal neurons early after neocortical trauma.

Lesioned neuronal circuits form new functional connections after a traumatic brain injury (TBI). In humans and animal models, aberrant excitatory connections that form after TBI may contribute to the pathogenesis of post-traumatic epilepsy. Partial neocortical isolation ("undercut" or "UC") leads to altered neuronal circuitry and network hyperexcitability recorded in vivo and in brain slices from chronically lesioned neocortex. Recent data suggest a critical period for maladaptive excitatory circuit formation within the first 3days post UC injury (Graber and Prince 1999, 2004; Li et al. 2011, 2012b). The present study focuses on alterations in excitatory connectivity within this critical period. Immunoreactivity (IR) for growth-associated protein (GAP)-43 was increased in the UC cortex 3days after injury. Some GAP-43-expressing excitatory terminals targeted the somata of layer V pyramidal (Pyr) neurons, a domain usually innervated predominantly by inhibitory terminals. Immunocytochemical analysis of pre- and postsynaptic markers showed that putative excitatory synapses were present on somata of these neurons in UC neocortex. Excitatory postsynaptic currents from UC layer V Pyr cells displayed properties consistent with perisomatic inputs and also reflected an increase in the number of synaptic contacts. Laser scanning photostimulation (LSPS) experiments demonstrated reorganized excitatory connectivity after injury within the UC. Concurrent with these changes, spontaneous epileptiform bursts developed in UC slices. Results suggest that aberrant reorganization of excitatory connectivity contributes to early neocortical hyperexcitability in this model. The findings are relevant for understanding the pathophysiology of neocortical post-traumatic epileptogenesis and are important in terms of the timing of potential prophylactic treatments.

The expression of kainate receptor subunits in hippocampal astrocytes after experimentally induced status epilepticus.

Astrocytes have emerged as active participants of synaptic transmission and are increasingly implicated in neurologic disorders including epilepsy. Adult glial fibrillary acidic protein (GFAP)-positive hippocampal astrocytes are not known for ionotropic glutamate receptor expression under basal conditions. Using a chemoconvulsive status epilepticus (SE) model of temporal lobe epilepsy, we show by immunohistochemistry and colocalization analysis that reactive hippocampal astrocytes express kainate receptor (KAR) subunits after SE. In the CA1 region, GluK1, GluK2/3, GluK4, and GluK5 subunit expression was observed in GFAP-positive astrocytes during the seizure-free or "latent" period 1 week after SE. At 8 weeks after SE, a time after SE when spontaneous behavioral seizures occur, the GluK1 and GluK5 subunits remained expressed at significant levels. Kainate receptor subunit expression was found in astrocytes in the hippocampus and surrounding cortex but not in GFAP-positive astrocytes of striatum, olfactory bulb, or brainstem. To examine hippocampal KAR expression more broadly, astroglial-enriched tissue fractions were prepared from dissected hippocampi and were found to have greater GluK4 expression after SE than controls. These results demonstrate that astrocytes begin to express KARs after seizure activity and suggest that their expression may contribute to the pathophysiology of epilepsy.

Targets for preventing epilepsy following cortical injury.

Prophylaxis of posttraumatic epilepsy will require a detailed knowledge of the epileptogenic pathophysiological processes that follow brain injury. Results from studies of experimental models and human epilepsy highlight alterations in GABAergic interneurons and formation of excessive new excitatory synaptic connectivity as prominent targets for prophylactic therapies. Promising laboratory results suggest that it will be possible to experimentally modify these aberrant processes and interfere with epileptogenesis. However, a number of key issues must be addressed before these results can be used to frame clinical antiepileptogenic therapy.

Abnormal cortical cells and astrocytomas in the Eker rat model of tuberous sclerosis complex.

PURPOSE: In patients with tuberous sclerosis complex (TSC), a wide range of neurologic abnormalities develop, including mental retardation and seizures. Brains from TSC patients are characterized by the presence of cortical tubers, large dysmorphic neurons, and abnormal cytomegalic cells. Although analysis of human TSC brain samples led to the identification of these abnormal cell types, very little is known about how these cells function. In an effort to model TSC-associated CNS abnormalities (and ultimately to analyze the electrophysiologic properties of abnormal cells), we examined Eker rats carrying a Tsc2 mutation. Anatomic studies, including standard histologic stains and immunocytochemistry, were performed on young Eker rats exposed to a carcinogen in utero or aged untreated Eker rats (18-24 months old). METHODS: Pregnant TSC2+/- females were injected once a day with hydroquinone (HQ), and offspring were killed at postnatal day P14 or P28. Coronal tissue sections throughout the CNS were prepared and stained for cresyl violet. In separate studies, brains of old untreated Eker rats were sectioned for anatomic analysis by using standard immunohistochemical techniques. RESULTS: Tissue sections stained with cresyl violet did not reveal any gross differences between HQ-treated Eker (Tsc2Ek/+) rats and siblings (Tsc2+/+). However, two classes of abnormal giant cells were observed in brain sections from untreated aged Eker rats: (a) large dysmorphic pyramid-like cells immunoreactive for NeuN, tuberin, and EAAC-1 in layers IV-VI; and (b) abnormal cytomegalic cells immunoreactive for glial fibrillary acidic protein (GFAP), vimentin, and nestin in deep cortical layers or along the white matter. In addition, large subependymal astrocytomas were observed in four animals. CONCLUSIONS: Our data suggest that cortical tuber formation in Eker rats is a rare event and that prenatal exposure to a nongenotoxic carcinogen such as HQ is not sufficient to induce tuber formation. However, with advanced age, an increased likelihood of astrocytoma formation and the emergence of dysmorphic neurons and cytomegalic cells in the Eker rat brain might exist; each of these abnormalities mimics those seen clinically and could contribute to neurologic problems associated with TSC. Further analysis of this rodent model may be warranted.